Depolarization
When polarized light is sent through an optical fiber, it undergoes depolarization. This happens both in the case of classical light as well as non-classical light, such as (but not limited to) squeezed states or single photons. Depolarization leads to limitations in communication applications, both in present day optical communication and also in future quantum communication. Reducing depolarization is an important task.
It is important to mention that for some applications it is enough to be able to maintain polarization only along one single basis (such as horizontal and vertical linear polarization, or right and left circular polarization). In other applications, such as those envisaged in quantum communication, it is important to be able to maintain polarization along all directions on the Bloch sphere; this is a far more demanding task.
There are several reasons why light undergoes depolarization while propagating through a fiber. First of all, the polarization may fluctuate randomly due to interactions with the environment. For instance defects in the fiber can induce random polarization rotations and random polarization losses. One can think of this as entanglement (in the quantum mechanical sense) of polarization and the environment.
Secondly, polarization will undergo rotation while propagating through the fiber. The rotation will, in general, be different for different frequencies, because different frequencies couple differently to polarization. This phenomenon is called Polarization Mode Dispersion (PMD). It can be thought of as entanglement (in the quantum mechanical sense) of polarization and frequency.
Signal Distortion by Polarization Mode Dispersion
While propagating through an optical fiber, an optical pulse changes shape, typically increasing its length. This is an extremely important effect since it can limit considerably the bit rate for classical communication over long fibers: pulses cannot be sent too close to each other but must be kept separated so that they do not overlap when they get distorted. There are many reasons why optical pulses get distorted during propagation through optical fibers. One of them is the polarization mode dispersion as discussed in the previous section. This effect has been studied for instance in [1] [2]. Limiting such distortion is of major importance in optical communication.
Methods for Correcting for Depolarization
There are a number of methods for limiting PMD. We refer to [3] for a recent review of these methods.
One way is to reduce as much as possible asymmetries and stresses in the fiber as it is drawn, thereby reducing the birefringence of the fiber.
Another method is based on spinning the fiber while it is being drawn [4][5]. This method was further studied, among other articles, in [6][7][8][9]. In particular in [6] different spinning profiles were compared, including constant spin, sinusoidal spin, Frequency Modulated spin, and Amplitude Modulated spin. This method does not reduce birefringence in the fiber, but rotates along the fiber the birefringence inducing defects. This reduces the overall effects of the birefringence defects on the polarization by essentially averaging out effects of the different birefringent regions of the fiber. However spinning cannot reduce PMD that arises due to circular birefringence, and it cannot reduce PMD which arises after fabrication, for instance due to stresses (twisting, bending) which occur during deployment.
Methods which attempt to overcome quantum noise in optical fibers are known from [12] and [13]. These publications suggest using phase-shifters and/or beam splitters to reduce the effects of noise (including changes in polarization direction) which occur in quantum communications systems. However, they address neither the problem of depolarization due to polarization mode dispersion, nor pulse spreading due to PMD.
Accordingly there is a need for improved methods for reducing depolarization in a transmission line, especially where such depolarization arises as a result of PMD. There is also a need for improved methods for reducing pulse spreading due to polarization mode dispersion. Embodiments of the present invention address one or more of these problems.